1. Field of the Invention
The present invention relates to apparatus for transforming a single frequency, linearly polarized laser beam into a beam with two, orthogonally polarized frequencies. More particularly, the invention relates to electro-optical apparatus which is useful in a variety of optical measuring devices which perform extremely accurate measurement of changes in either length or optical length.
2. The Prior Art
The use of optical interferometry to measure changes in either length, distance, or optical length has grown significantly due not only to technological advances in lasers, photosensors, and microelectronics but also to an ever increasing demand for high precision, high accuracy measurements. The prior-art interferometers can be generally categorized into two types based on the signal processing technique used, i.e., either homodyne or heterodyne. The interferometers based on the heterodyne technique are generally preferred because (1) they are insensitive to low frequency drift and noise and (2) they can more readily have their resolution extended. Within the heterodyne type of interferometers, of particular interest are the ones based on the use of two optical frequencies. In the prior-art, two-optical frequency heterodyne interferometers, the two optical frequencies are produced by one of the following techniques: (1) use of a Zeeman split laser, see for example, Bagley et al, U.S. Pat. No. 3,458,259 issued July 29, 1969; G. Bouwhuis, "Interferometrie Met Gaslasers," Ned. T. Natuurk, vol. 34, pp. 225-232 (August 1968); Hewlett Packard Journal (August 1970); Bagley et al., U.S. Pat. No. 3,656,853 issued April 18, 1972; Hewlett-Packard Journal (Apr. 1983); and H. Matsumoto, "Recent interferometric measurements using stabilized lasers," Precision Engineering, vol. 6, pp. 87-94 (Apr. 1984); (2) use of a pair of acousto-optic Bragg cells, see for example, Y. Ohtsuka and K. Itoh, "Two-frequency laser interferometer for small displacement measurements in a low frequency range," Applied Optics, vol. 18, pp. 219-224 (15 January 1979); N. Massie et al., "Measuring laser flow fields with a 64-channel heterodyne interferometer," Applied Optics, vol. 22, pp. 2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, "Dynamic two-frequency interferometry for small displacement measurements," Optics and Laser Technology, vol. 16, pp. 25-29 (February 1984); H. Matsumoto, op.cit or, (3) use of the two longitudinal modes of a randomly polarized HeNe laser, see for example, J. B. Ferguson and R. H. Morris, "Single Mode Collapse in 6328-A He-Ne lasers," Applied Optics, vol. 17, pp. 2924-2929 (1978).
The use of a Zeeman split laser to produce the two optical frequencies is only applicable to certain lasers (e.g., HeNe) and limits the frequency difference between the two optical frequencies to about 2 MHz. This imposes a limit on the maximum rate of change of the length or optical length being measured. In addition, the available power from a Zeeman split laser is less than 500 microwatts which can be a serious limitation when one laser source must be used for the measurement for multiple axes, e.g., 3-6 axes. Another limitation of the Zeeman split laser is that the frequency difference depends upon external applied magnetic fields which can vary with time, location, and orientation e.g., the earth's magnetic field or the magnetic fields produced by electrical currents in nearby electrical equipment. Therefore, it is necessary to measure this uncontrolled variability of the frequency difference in any measuring apparatus which relies on the constancy of the difference frequency.
The use of two Bragg cells to produce the two optical frequencies requires complex, expensive apparatus which is susceptible to a number of sources of error and alignment difficulties.
The use of two longitudinal modes of a randomly polarized HeNe laser provides a laser beam with two orthogonally polarized frequencies in a rather convenient, cost-effective form. However, the frequency difference is approximately 600-1000 MHz which requires complicated, expensive detection and processing electronics. Furthermore, by starting with such a high frequency difference, the task of resolution extension becomes difficult and expensive.
While prior-art techniques for producing a laser beam with two optical frequencies of orthogonal polarization are useful for some applications, none provide the technical performance in a commercially viable form for applications requiring the measurement of rapidly changing lengths (distances) to extremely high resolution.